K. Popper - Logic scientific discovery
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K. Popper - Logic scientific discovery

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5 and 6, and end of *52.)

some structural components of a theory of experience80

(There is a widespread belief that the statement ‘I see that this table
here is white’, possesses some profound advantage over the statement
‘This table here is white’, from the point of view of epistemology. But
from the point of view of evaluating its possible objective tests, the first
statement, in speaking about me, does not appear more secure than the
second statement, which speaks about the table here.)

There is only one way to make sure of the validity of a chain of
logical reasoning. This is to put it in the form in which it is most easily
testable: we break it up into many small steps, each easy to check by
anybody who has learnt the mathematical or logical technique of trans-
forming sentences. If after this anybody still raises doubts then we can
only beg him to point out an error in the steps of the proof, or to think
the matter over again. In the case of the empirical sciences, the situation
is much the same. Any empirical scientific statement can be presented
(by describing experimental arrangements, etc.) in such a way that
anyone who has learned the relevant technique can test it. If, as a result,
he rejects the statement, then it will not satisfy us if he tells us all about
his feelings of doubt or about his feelings of conviction as to his
perceptions. What he must do is to formulate an assertion which con-
tradicts our own, and give us his instructions for testing it. If he fails to
do this we can only ask him to take another and perhaps a more careful
look at our experiment, and think again.

An assertion which owing to its logical form is not testable can at
best operate, within science, as stimulus: it can suggest a problem. In
the field of logic and mathematics, this may be exemplified by Fermat’s
problem, and in the field of natural history, say, by reports about sea-
serpents. In such cases science does not say that the reports are
unfounded; that Fermat was in error or that all the records of observed
sea-serpents are lies. Instead, it suspends judgment.3

Science can be viewed from various standpoints, not only from that
of epistemology; for example, we can look at it as a biological or as a
sociological phenomenon. As such it might be described as a tool, or
an instrument, comparable perhaps to some of our industrial
machinery. Science may be regarded as a means of production—as the

3 Cf. the remark on ‘occult effects’ in section 8.

the problem of the empirical basis 81

last word in ‘roundabout production’.4 Even from this point of view
science is no more closely connected with ‘our experience’ than other
instruments or means of production. And even if we look at it as
gratifying our intellectual needs, its connection with our experiences
does not differ in principle from that of any other objective structure.
Admittedly it is not incorrect to say that science is ‘. . . an instrument’
whose purpose is ‘. . . to predict from immediate or given experiences
later experiences, and even as far as possible to control them’.5 But I do
not think that this talk about experiences contributes to clarity. It has
hardly more point than, say, the not incorrect characterization of an oil
derrick by the assertion that its purpose is to give us certain experi-
ences: not oil, but rather the sight and smell of oil; not money, but
rather the feeling of having money.


It has already been briefly indicated what rôle the basic statements play
within the epistemological theory I advocate. We need them in order to
decide whether a theory is to be called falsifiable, i.e. empirical. (Cf.
section 21.) And we also need them for the corroboration of falsifying
hypotheses, and thus for the falsification of theories. (Cf. section 22.)

Basic statements must therefore satisfy the following conditions. (a)
From a universal statement without initial conditions, no basic state-
ment can be deduced.*1 On the other hand, (b) a universal statement

4 The expression is Böhm-Bawerk’s (‘Produktionsumweg’).
5 Frank, Das Kausalgesetz und seine Grenzen, 1932, p. 1. *Concerning instrumentalism, see note
*1 before section 12, and my Postscript, especially sections *12 to *15.
*1 When writing this, I believed that it was plain enough that from Newton’s theory
alone, without initial conditions, nothing of the nature of an observation statement can
be deducible (and therefore certainly no basic statements). Unfortunately, it turned out
that this fact, and its consequences for the problem of observation statements or ‘basic
statements’, was not appreciated by some of the critics of my book. I may therefore add
here a few remarks.

First, nothing observable follows from any pure all-statement—‘All swans are white’,
say. This is easily seen if we contemplate the fact that ‘All swans are white’ and ‘All swans
are black’ do not, of course, contradict each other, but together merely imply that there
are no swans—clearly not an observation statement, and not even one that can be
‘verified’. (A unilaterally falsifiable statement like ‘All swans are white’, by the way, has

some structural components of a theory of experience82

and a basic statement can contradict each other. Condition (b) can only
be satisfied if it is possible to derive the negation of a basic statement
from the theory which it contradicts. From this and condition (a) it
follows that a basic statement must have a logical form such that its
negation cannot be a basic statement in its turn.

We have already encountered statements whose logical form is
different from that of their negations. These were universal state-
ments and existential statements: they are negations of one another,
and they differ in their logical form. Singular statements can be con-
structed in an analogous way. The statement: ‘There is a raven in the
space-time region k’ may be said to be different in its logical form—
and not only in its linguistic form—from the statement ‘There is no
raven in the space-time region k’. A statement of the form ‘There is a
so-and-so in the region k’ or ‘Such-and-such an event is occurring in
the region k’ (cf. section 23) may be called a ‘singular existential
statement’ or a ‘singular there-is statement’. And the statement which
results from negating it, i.e. ‘There is no so-and-so in the region k’ or
‘No event of such-and-such a kind is occurring in the region k’, may

the same logical form as ‘There are no swans’, for it is equivalent to ‘There are no
non-white swans’.)

Now if this is admitted, it will be seen at once that the singular statements which can be
deduced from purely universal statements cannot be basic statements. I have in mind
statements of the form: ‘If there is a swan at the place k, then there is a white swan at the
place k.’ (Or, ‘At k, there is either no swan or a white swan.’) We see now at once why
these ‘instantial statements’ (as they may be called) are not basic statements. The reason
is that these instantial statements cannot play the role of test statements (or of potential falsifiers)
which is precisely the role which basic statements are supposed to play. If we were to
accept instantial statements as test statements, we should obtain for any theory (and
thus both for ‘All swans are white’ and for ‘All swans are black’) an overwhelming
number of verifications—indeed, an infinite number, once we accept as a fact that the
overwhelming part of the world is empty of swans.

Since ‘instantial statements’ are derivable from universal ones, their negations must be
potential falsifiers, and may therefore be basic statements (if the conditions stated below
in the text are satisfied). Instantial statements, vice versa, will then be of the form of
negated basic statements (see also note *4 to section 80). It is interesting to note that
basic statements (which are too strong to be derivable from universal laws alone) will
have a greater informative content than their instantial negations; which means that the
content of basic statements exceeds